The present invention relates generally to the detection and measurement of transmembrane potentials. In particular, the present invention is directed to compositions and optical methods for determining transmembrane potentials across the plasma membrane of biological cells.
This invention was made with Government support under Grant No. R01 NS27177-07, awarded by the National Institutes of Health. The Government has certain rights in this invention.
Fluorescence detection and imaging of cellular electrical activity is a technique of great importance and potential (Grinvald, A., Frostig, R. D., Lieke, E., and Hildesheim, R. 1988. Optical imaging of neuronal activity. Physiol. Rev. 68:1285-1366; Salzberg, B. M. 1983. Optical recording of electrical activity in neurons using molecular probes. In Current Methods in Cellular Neurobiology. J. L. Barker, editor. Wiley, New York. 139-187; Cohen, L. B. and S. Lesher. 1985. Optical monitoring of membrane potential: methods of multisite optical measurement. In Optical Methods in Cell Physiology. P. de Weer and B. M. Salzberg, editors. Wiley, New York. 71-99).
Mechanisms for optical sensing of membrane potential have traditionally been divided into two classes:
(1) sensitive but slow redistribution of permeant ions from the extracellular medium into the cell, and
(2) fast but small perturbations of relatively impermeable dyes attached to one face of the plasma membrane. see, Loew, L. M., xe2x80x9cHow to choose a potentiometric membrane probexe2x80x9d, In Spectroscopic Membrane Probes. L. M. Loew, ed., 139-151 (1988) (CRC Press, Boca Raton); Loew, L. M., xe2x80x9cPotentiometric membrane dyesxe2x80x9d, In Fluorescent and Luminescent Probes for Biological Activity. W. T. Mason, ed., 150-160 (1993) (Academic Press, San Diego).
The permeant ions are sensitive because the ratio of their concentrations between the inside and outside of the cell can change by up to the Nernstian limit of 10-fold for a 60 mV change in transmembrane potential. However, their responses are slow because to establish new equilibria, ions must diffuse through unstirred layers in each aqueous phase and the Low-dielectric-constant interior of the plasma membrane. Moreover, such dyes distribute into all available hydrophobic binding sites indiscriminately. Therefore, selectivity between cell types is difficult. Also, any additions of hydrophobic proteins or reagents to the external solution, or changes in exposure to hydrophobic surfaces, are prone to cause artifacts. These indicators also fail to give any shift in fluorescence wavelengths or ratiometric output. Such dual-wavelength readouts are useful in avoiding artifacts due to variations in dye concentration, path length, cell number, source brightness, and detection efficiency.
By contrast, the impermeable dyes can respond very quickly because they need little or no translocation. However, they are insensitive because they sense the electric field with only a part of a unit charge moving less than the length of the molecule, which in turn is only a small fraction of the distance across the membrane. Furthermore, a significant fraction of the total dye signal comes from molecules that sit on irrelevant membranes or cells and that dilute the signal from the few correctly placed molecules.
In view of the above drawbacks, methods and compositions are needed which are sensitive to small variations in transmembrane potentials and can respond both to rapid, preferably on a millisecond timescale, and sustained membrane potential changes. Also needed are methods and compositions less susceptible to the effects of changes in external solution composition, more capable of selectively monitoring membranes of specific cell types, and providing a ratiometric fluorescence signal. This invention fulfils this and related needs.
Methods and compositions are provided for determining transmembrane electrical potential (membrane potential), particularly across the outermost (plasma) membrane of living cells. In one aspect, the method comprises:
(a) introducing a first reagent comprising a hydrophobic fluorescent ion, which is capable of redistributing from a first face of the membrane to a second face of the membrane in response to changes in the potential of the membrane, as described by the Nernst equation,
(b) introducing a second reagent which labels one face, usually the extracellular face of the membrane, which second reagent comprises a chromophore, capable of undergoing energy transfer by either (i) donating excited state energy to the fluorescent ion, or (ii) accepting excited state energy from the fluorescent ion,
(c) exposing the membrane to excitation light of an appropriate wavelength, typically in the ultraviolet or visible region;
(d) measuring energy transfer between the fluorescent ion and the second reagent, and
(e) relating the extent of energy transfer to the membrane potential.
The second reagent is preferably a fluorophore. In each case the excited state interaction can proceed by fluorescence resonance energy transfer (FRET), which is preferred, or some other mechanism such as electron transfer, exchange (Dexter) interaction, paramagnetic quenching, or promoted intersystem crossing. The method finds particular utility in detecting changes in membrane potential of the plasma membrane in biological cells.
Preferably, the hydrophobic ion is an anion which labels the extracellular face of the plasma membrane. Upon addition of the hydrophobic fluorescent anion to the membrane, cell, or tissue preparation, the anion partitions into the plasma membrane, where it distributes between the extracellular and intracellular surfaces according to a Nernstian equilibrium. Changes in the membrane potential cause the fluorescent anion to migrate across the membrane so that it can continue to bind to whichever face (the intracellular or extracellular face) is now positively charged. Since the efficiency of energy transfer between the two reagents is a function of the distance between them and this distance varies as the fluorescent anion redistributes back and forth across the membrane, measurement of energy transfer provides a sensitive measure of changes in the transmembrane potential. For example, if the membrane potential (intracellular relative to extracellular) changes from negative to positive, the fluorescent hydrophobic anion is pulled from the extracellular surface to the intracellular surface of the plasma membrane. If the second reagent is one which is on the extracellular face, this results in an increase in the distance between the anion and the second reagent and a concomitant decrease in the efficiency of FRET and quenching between the two species. Thus, fluorescence measurements at appropriate excitation and emission wavelengths provide a fluorescent readout which is sensitive to the changes in the transmembrane potential. Typically, the time constant for the redistribution of the fluorescent anion is rapid and in the millisecond time scale thus allowing the convenient measurement both of rapid cellular electrical phenomena such as action potentials or ligand-evoked channel opening, as well as slower and more sustained changes evoked by altering the activity of ion pumps or exchangers.
Conventional electrophysiological techniques read the potential at the tip of an electrode and are thus limited to measurements of a single cell. By contrast, the optical indicators described herein are particularly advantageous for monitoring the membrane potential of many neurons or muscle cells simultaneously. Optical indicators, unlike conventional microelectrodes, do not require physical puncture of the membrane; in many cells or organelles, such puncture is highly injurious or mechanically difficult to accomplish. Optical indicators are thus suitable for cells too small or fragile to be impaled by electrodes.
In another aspect of the invention, the voltage sensing methods allow one to detect the effect of test samples, such as potential therapeutic drug molecules, on the activation/deactivation of ion transporters (channels, pumps, or exchangers) embedded in the membrane.
The compositions of the present invention comprise two reagents. The first reagent comprises a hydrophobic fluorescent ion (preferably an anion) which is capable of redistributing from one face of a membrane to the other in response to changes in transmembrane potential. This anion is referred to as the mobile or hydrophobic anion. Exemplary anions are polymethine oxonols, tetraaryl borates conjugated to fluorophores and fluorescent complexes of rare earth and transition metals. The second reagent comprises a chromophore, preferably a fluorophore, capable of undergoing energy transfer by either (i) donating excited state energy to the fluorescent anion, or (ii) accepting excited state energy from the fluorescent anion. The second reagent binds selectively to one face of the membrane, and unlike the first reagent, does not redistribute in response to transmembrane potential changes. Therefore, it is referred to as the asymmetrically bound or immobile reagent. Exemplary second reagents include fluorescent lectins, fluorescent lipids, fluorescent carbohydrates, fluorescently labelled antibodies against surface membrane constituents, and amphiphilic fluorescent dyes. In certain preferred embodiments of the invention, the first and second reagents are bound together by a suitable flexible linker group. The linker group is long enough to permit the first reagent to reside in the opposite face of the membrane from the second reagent and reduce FRET.